In recent years, commercial applications of LEDs have been continuously expanding. In particular, LEDs are increasingly used for white light illumination because of the awareness of global warming. LED packaging is an important aspect for LED applications. Among other things, the functions of the LED packaging include insuring proper electrical connections to the anode and cathode of the LED, and protecting the LED chip from mechanical, thermal, and humidity impact from the environment. Selecting an LED packaging method for a particular application depends on the physical shape and electrical characteristics of the LED, as well as their optical properties. As new applications of LEDs are being developed, there are also new challenges in packaging methods. As an important aspect of the LED packaging, die-bonding is perhaps the first challenge that one faces for high power LED applications.
There are two die-bonding methods that are widely used in the LED industry: epoxy die bonding and eutectic die-bonding. FIG. 4 illustrates the method of the epoxy die-bonding. In step 302, an amount of a first epoxy 342 is applied to the bottom of a packaging cup 324 using a dispenser 340. In step 304, an LED chip 320 is placed into the packaging cup 324. In step 306, the back side of the LED chip 320 is pressed into the epoxy 342. The assembly, including the packaging cup 324, the LED chip 320, and the epoxy 342, is baked at approximately 150° C. for about one to two hours to cure the first epoxy thereby to fix the LED chip 320 in the packaging cup. In step 308, a first conductive wire 328 is bonded to the anode of the LED at one end thereof and to a first electrode (not shown) at the other end thereof. A second conductive wire 329 is bonded to the cathode of the LED at one end thereof and to a second electrode (not shown) at the other end thereof. The first electrode and the second electrode are attached to the packaging cup 324. At step 310, the packaging cup is filled with a second epoxy 330 to immerse the LED chip 320, the first conductive wire 328, and the second conductive wire 329 therein. The assembly is then baked again to cure the second epoxy.
The epoxy die-bonding is widely used for low power LEDs in small size products or backlight products. The first epoxy used to attach the LED chip to the packaging cup can be either epoxy resin or silicon epoxy, each of which has a thermal conductivity coefficient around 5 W/mK. In some products, for example illumination products or quarternary products, some silver power is added into the first epoxy to improve its electrical and thermal conductivity. Silver epoxy can have a thermal conductivity coefficient of about 20 W/mK at most. The main disadvantages of the epoxy die-bonding are the relatively low thermal conductivity it can achieve and the relatively long baking time required for curing the first epoxy.
For applications using high power LEDs, new die-bonding techniques are developed. Among them is a eutectic die-bonding method. FIG. 5 illustrates the method of the eutectic die-bonding. In step 402, the back side of an LED chip 420 is coated with a thin metal layer 422 such as gold (Au), tin (Sn), or Au—Sn alloy. In step 404, the LED chip 420 is placed into a packaging cup 424. The interior surface of the bottom of the packaging cup 424 is coated with a metal such as Au or silver (Ag). In step 406, the LED chip 420 is positioned in the packing cup 424 such that the back side of the LED chip 420 is in contact with the bottom of the packaging cup 424. The assembly, including the packaging cup 424 and the LED chip 420, is heated to a temperature above about 230° C. As the Au or Ag coated on the bottom of the packaging cup 424 melts and penetrates into the metal layer 422 on the back side of the LED chip 420, the melting point of the alloy increases causing the alloy to solidify, thereby fixing the LED chip 420 in the packaging cup 424. In step 408, conductive wires 428 and 429 are bonded to the anode and cathode of the LED, respectively. In step 410, the packaging cup is filled with an epoxy 430. The assembly is then baked at an elevated temperature for a duration of time effective to cure the epoxy. The eutectic die-bonding can achieve a thermal conductivity coefficient as high as 60 W/mK, far superior than what the epoxy die-bonding can achieve. However, the eutectic die-bonding involves relatively complicated processes and requires expensive equipment. In addition, the thermal properties of the LED chip and the substrate also need to be taken into consideration.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.